The present invention relates generally to optical scanning devices and, more particularly, to a catadioptric lens system for an optical scanning device.
Optical scanning devices are well-known in the art and produce machine-readable data which is representative of the image of an object, e.g., a page of printed text. Optical scanning devices generally employ line-focus systems which image an object by sequentially focusing narrow xe2x80x9cscan linexe2x80x9d portions of the object onto a linear photosensor array by sweeping a scanning head over the object.
In a line-focus system, a light beam from an illuminated line object is imaged by a lens on a linear photosensor array which is positioned remotely from the line object. The linear photosensor array is a single dimension array of photoelements which correspond to small area locations on the line object. These small area locations on the line object are commonly referred to as xe2x80x9cpicture elementsxe2x80x9d or xe2x80x9cpixels.xe2x80x9d In response to light from its corresponding pixel location on the line object, each photosensor pixel element in the linear photosensor array (sometimes referred to simply as a xe2x80x9cpixelxe2x80x9d) produces a data signal which is representative of the light intensity that it experiences during an immediately preceding interval of time known as a sampling interval. All of the photoelement data signals are received and processed by an appropriate data processing system.
In a color optical scanning device, a plurality of spectrally separated imaging beams (typically red, green and blue beams) must be projected onto photosensor arrays. Some color optical scanning devices employ beam splitter devices for spectrally separating an imaging light beam into color component beams. These separate color component beams are projected onto separate linear photosensor arrays. Other optical scanning devices project color component images on a single linear array in a series of separate scanning passes.
The construction and operation of color optical scanning devices employing beam splitter assemblies and photosensor arrays are disclosed in the following United States Patents: U.S. Pat. No. 5,410,347 of Steinle et al. for COLOR OPTICAL SCANNER WITH IMAGE REGISTRATION HOLDING ASSEMBLY; U.S. Pat. No. 4,870,268 of Vincent et al. for COLOR COMBINER AND SEPARATOR AND IMPLEMENTATIONS; U.S. Pat. No. 4,926,041 of Boyd for OPTICAL SCANNER (and corresponding EPO patent application no. 90306876.5 filed Jun. 22, 1990); U.S. Pat. No. 5,019,703 of Boyd et al. for OPTICAL SCANNER WITH MIRROR MOUNTED OCCLUDING APERTURE OR FILTER (and corresponding EPO patent application no. 90312893.2 filed Nov. 27, 1990); U.S. Pat. No. 5,032,004 of Steinle for BEAM SPLITTER APPARATUS WITH ADJUSTABLE IMAGE FOCUS AND REGISTRATION (and corresponding EPO patent application no. 91304185.1 filed May 9, 1991); U.S. Pat. No. 5,044,727 of Steinle for BEAM SPLITTER/COMBINER APPARATUS (and corresponding EPO patent application no. 91303860.3 filed Apr. 29, 1991); U.S. Pat. No. 5,040,872 of Steinle for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR (and corresponding EPO patent application no. 90124279.2 filed Dec. 14, 1990 which has been abandoned); and U.S. Pat. No. 5,227,620 of Elder, Jr. et al. for APPARATUS FOR ASSEMBLING COMPONENTS OF COLOR OPTICAL SCANNERS (and corresponding EPO patent application no. 91304403.8 filed May 16, 1991), which are all hereby specifically incorporated by reference for all that is disclosed therein.
A hand-held optical scanning device is an optical scanner which is moved across a scanned object, e.g. a page of text, by hand. Rollers may be provided on a hand-held scanning device to guide the device across the object to be scanned and also to provide data to the scanning device microprocessor regarding the speed at which the scanning device is being moved over the scanned object. These rollers may also serve to control the speed at which an operator moves the scanning device across the scanned object.
The construction and operation of hand-held optical scanning devices employing such rollers is disclosed in United States patents: U.S. Pat. No. 5,381,020 of Kochis et al. for HAND-HELD OPTICAL SCANNER WITH ONBOARD BATTERY RECHARGING ASSEMBLY and U.S. Pat. No. 5,306,908 of McConica et al. for MANUALLY OPERATED HAND-HELD OPTICAL SCANNER WITH TACTILE SPEED CONTROL ASSEMBLY (and corresponding EPO patent application no. 94301507.3 filed Mar. 2, 1994), and in U.S. patent application Ser. No. 08/601,276 filed Jan. 29, 1996 of Kerschner et al. for HAND-HELD SCANNING DEVICE; U.S. patent application Ser. No. 08/592,904 filed Jan. 29, 1996 of Kerschner et al. for SCANNING DEVICE WITH NON-CONTACT OPTICAL COMPONENTS; U.S. patent application Ser. No. 08/878,110 filed Jun. 18, 1997, of Kerschner et. al. for SCANNING DEVICE WITH FLOATING WINDOW MEMBER; and U.S. patent application Ser. No. 08/878,429 filed Jun. 18, 1997, of Kerschner et al. for ILLUMINATION SYSTEM WITH WHITE LEVEL CALIBRATION FOR HAND-HELD SCANNER which are all hereby specifically incorporated by reference for all that is disclosed therein.
In a typical scanning device, a lens is generally provided which separates the light beam into an object path portion and an image path portion. The object path portion generally extends between the object being scanned and the lens while the image path portion generally extends between the lens and the photosensor array. In order to scan conventional size documents, most scanning devices have a length of at least about 8.5 inches. A typical linear photosensor array, however, may have a length of only about 1.21 inches. The imaging assembly of a scanning device, thus must be configured to reduce the scan line image to the size of the photosensor array, e.g., from about 8.5 inches to about 1.21 inches.
As is well known, the amount of image reduction caused by an imaging assembly is dictated by the relationship between the length of the object path and the length of the image path. Further, for a lens having a given focal length, the length of the object path and of the image path will be determined by the required image reduction. Accordingly, to achieve a given image reduction using a lens having a given focal length, the overall length of the imaging path must be a certain length. For example, if a lens having a focal length of 0.984 inches is used and an image reduction ratio of 7:1 is desired (as needed, e.g., to reduce a 8.5 inch long scan line to a 1.21 inch long photosensor array), then the length of the object path 50 must be about 7.87 inches and the length of the image path 52 must be about 1.125 inches. Thus, the overall length of the imaging path must be the sum of the object path and the image path lengths, or 8.995 inches.
The relationships set forth above dictate the geometry and physical size of the optical assembly of a conventional optical scanning device. Specifically, the necessity to maintain a light path having a particular length serves to limit the minimum size of the optical assembly and reduces the degree of compactness achievable for the imaging assembly and, thus, for the overall optical scanning device.
It is noted that it is possible to shorten the light path of an optical scanning device by using a shorter focal length lens. A shorter focal length lens, however, requires a greater field of view than a longer focal length lens. This greater field of view, in turn, worsens the optical aberrations, e.g., spherical aberration, which are inherent in lenses. Accordingly, it is not generally desirable to shorten the light path of an optical scanning device by merely reducing the focal length of the lens.
Optical systems for hand-held scanning devices must generally be very compact due to the relatively small size of hand-held scanning devices. Generally, such optical systems include various mirrors, and prisms to fold the light path in order to achieve the necessary optical path length in the smallest physical package feasible. Even with the use of such mirrors and prisms, however, the compactness of optical scanning devices is limited by the optical requirements set forth above.
The present invention is directed to an optical system for a scanning device. The optical system employs a catadioptric lens which both refracts and reflects the light passing through it. In this manner, the majority of the image path portion of the light beam may be folded within the lens. This enables the required optical path length to be achieved while providing a smaller, more compact physical envelope for the imaging assembly.
The catadioptric lens achieves focusing of the light beam through the use of mirrored surfaces on the lens. Several refractive surfaces are also provided to correct for various aberrations, such as, for example, spherical aberration.